Intruder detector systems are used to raise an alarm when an intruder passes into a detection zone. For example, a pair of spaced leaky coaxial cable antennae may be buried around a prison or a military air field. An RF signal applied to one antenna is received by the other antenna. A person entering into the RF field disturbs the field, and this disturbance can be detected by equipment connected to the receiving antenna, which equipment senses phase and/or amplitude changes in the received RF field relative to the transmitted signal.
Such intruder detector systems can be made in several forms, such as those that use a single length of antenna, those that divide the perimeter to be guarded into blocks, etc., and are not limited to use of RF signals. For example, some intruder detector system systems use vibration detectors attached to fences, windows or other structures that can be crossed, etc., separately or in combination with other detectors.
Examples of such intruder detector systems are described in U.S. Pat. Nos. 4,562,428 dated Dec. 31, 1985, 4,994,785 dated Feb. 19, 1991 and 4,887,069 dated Dec. 12, 1991, assigned to Senstar Corporation.
Such intruder detector systems are often affected by rain. For example, rain can increase the moisture content of the soil, changing the dielectric constant of a medium which carries an RF field which is to be detected. Rain and wind can cause vibration detectors to raise an alarm. Intruder detector system systems can be affected by humidity, lightning and electromagnetic interference (EMI), as well as rain and wind. These factors can cause detrimental operation of the intruder detector systems, increasing or decreasing their sensitivity, causing false alarms, etc.
Various techniques have been used to reduce the sensitivity of such systems to the environment, which fall into several categories: (a) detection of the output of each detector of the detector system at a central location and modification of an alarm indication threshold at the central location, with output signals of the sensors which are in excess of the threshold causing an alarm, and (b) variation of an alarm threshold at a local detector by local sensing of a noise factor.
However, neither of these cases provides optimum operation. For example, in the first case the central processor may be required to perform the detection signal processing for all of the individual sensors, which requires an extremely complex centralized algorithm. Further, signals from each sensor must be sent to the central processor, which is inefficient and wastes processing power. Thus if a sensor detects an "out of range" signal biased by the environment, it sends this redundant information to the central processor. In the second case, it is not possible to accurately determine that the sensor signal is caused by a common environmental stimulus, since each detector acts alone. Thus a detector may be purposely desensitized by a determined intruder prior to entry into the detection zone.
For example, in U.S. Pat. No. 4,857,912, plural sensors provide "on" and "off" inputs to a CPU, which weights the inputs from the sensors, averages them, and establishes a threshold above which an intrusion is declared. While this reference teaches that the operating parameters of the CPU can be changed by adjusting the weighting factors under varying conditions for sensors, and can vary the alarm threshold, it does not describe varying the parameters of individual sensors as a result of detecting environmental conditions. "On" and "off" data from the sensors is sent to the CPU, and all weighting is done at that central location, acting on all of the data arriving from each sensor.
In U.S. Pat. No. 4,977,527 the signals returned to a CPU from plural sensors are biased by a sensitivity level stored at the CPU. Each sensor is calibrated by sending a control signal to it and measuring its response. The result, stored at the CPU, is used to evaluate the signals received from the sensors, both as to value and as to history. However this system does not take drift data from plural sensors and use that as a calibration parameter, either to a central processor or to each sensor itself. A linear response curve is generated for each sensor which, when used with sensitivity data, produces a threshold alarm level for each detector, ensuring that each detector responds uniformly to ambient conditions. However, the sensitivity of each sensor is not corrected to compensate for environmental conditions.
In U.S. Pat. No. 5,170,359, plural sensors pass continuous sensor data through transient episode detector processors, which provide transient data to a central memory for processing by a central processor. Each of the transient episode detector processors individually adapt to ignore environmental variation data not of interest, e.g. wind noise, etc.
This system does not vary the transient episode detector processors' environmental data sensitivity generated by other sensors, but only from the data input from an associated sensor. Thus it is not possible to know whether the stimulus is an environmental condition affecting all detectors, or not.
U.S. Pat. No. 5,267,180 describes a fire alarm system in which a local CPU is associated with each fire sensor. Sensor data is transmitted to a central fire receiver after processing in the local CPU. The central fire receiver contains a central CPU which indicates a fire if the data from the local CPUs meet certain rules, which include weightings. The rules are stored centrally and the weightings are applied centrally, to obtain a fire likelihood value. However there is no feedback to the local CPUs of common environment data which desensitizes or biases it against the generation of signals resulting from common environment changes.
U.S. Pat. No. 3,947,834 describes an intruder detector system that includes a sensor which samples external noise parameters such as fence vibration, wind speed, rain rate, etc., and through an AGC desensitizes the system to intruder signals. However this system is comprised of only a single intruder detector processing system which is desensitized.
U.S. Pat. No. 5,465,080 describes plural single (infrared) sensors which produce background signals which are each integrated over time to obtain a "noise" signal, which is compared to a sensor output signal to determine a threshold signal. A sensor signal exceeding the threshold indicates the presence of an intruder. There is no cumulative sensor environmental desensitization.